Approximately 60% of the world's petroleum production derives from offshore operations. To meet demands, and in an environment of increased values for oil and gas, exploration and subsequent production is being undertaken in deeper and deeper waters. For example, oil and gas is now being produced in 5,000-10,000 feet of water. These offshore efforts have required expensive specialized solutions including establishment of extensive networks of subsea pipelines for transport of oil and gas from well heads to gathering structures, hub facilities and to onshore processing refineries.
Subsea pipeline undergo servicing throughout their lifetimes, beginning when they are first commissioned, intermittently during active service and as a consequence of temporary shut-ins, and when they are finally decommissioned. Newly constructed pipelines must undergo a series of commissioning steps to ensure that specifications have been met as well as to comply with regulatory requirements. Such requirements vary with pipeline composition and intended use.
Pipeline commissioning will generally include at least flooding, cleaning and gauging, hydrotesting, dewatering and drying before any product can be introduced into the line. The initial flooding operations typically include pushing of a preinstalled pig through the pipeline with treated seawater. The pipeline will be flooded between a pair of valved closures such as pipeline end terminations (PLETs). The pig or pigs transiting the pipeline in conjunction with flooding may participate in cleaning of the pipeline of millscale and other debris as well as assessment of dents, buckles and other out of round defects in the pipeline.
After completion of flooding, the pipeline is hydrostatically tested to federal guidelines by pressurizing the relatively non-compressible seawater filling the pipeline using high-pressure pumps. US Federal Safety Regulations set out at 49 C.F.R. Part 195, require “pipelines used to transport hazardous or highly volatile liquids” are to “be tested at a pressure equal to 125% of the maximum allowable operating pressure (MAOP)” for a set period. Gas pipeline hydrostatic testing requirements are set out in ASME B31.8 and API RP1110.
After hydrostatic testing, further cleaning may be undertaken such as for example brush pigging. Ultimately, whether seawater or potable water, the water must be removed from the pipeline. For large subsea pipelines, the volume of water that must be removed can be considerable. For one example, dewatering of the 134 miles of 20/24 dual diameter Independence Trail Gas Export Pipeline in the Gulf of Mexico required removal of thirteen million gallons of seawater.
Dewatering is typically conducted by pushing of dewatering pigs from one terminus to the other using compressed air, nitrogen, or the like. Given the immense volume to be removed, the dewatering effort may take several weeks of continuous pumping. After dewatering, the pipeline is conditioned. Water removal is required to prevent contamination of product as well as to avoid corrosion. Water removal and subsequent chemical treatment of the pipeline is particularly important to avoid hydrate formation in natural gas pipelines.
Physical removal of water is typically accomplished by sweeping water out of the line using a pig train pushed by a gas such as air or nitrogen. Salt may be flushed from the line using slugs of fresh water batched between pairs of pigs in the front of the pig train. Chemical treatment or swabbing is typically employed in natural gas pipelines to prevent hydrate formation. Chemical swabbing involves traversing slugs of hydroscopic chemicals, commonly methanol or monoethylene glycol (MEG), through the pipeline between sets of pigs that together form a pig train. For example, slugs of methanol or MEG may be placed in two or more batches in a train of four or five pigs, driven by the pressure of compressed gas. In the case of methanol swabbing, slugs of methanol may be interspersed with slugs of nitrogen to lessen risks of explosion.
Chemical treatment solutions cannot be released into the environment and heretofore have been collected through conduits running to temporary storage tanks on surface vessels. Therefore, the swabbing chemicals are collected using tubing that runs from the seafloor to a surface vessel or platform for collection. The subsea removal of chemical treatment solutions has not been heretofore possible and represents an unmet need in the industry.
What are needed are subsea pipeline service apparatus and methods able to provide for subsea collection of subsea pipeline chemicals from flowline system elements.